Resiliency: mark a service as used even if the getClient call during warmUp fails

[ZoabKapoor]

Summary

We recently observed a bug with some users of D2 where:

1. During warmUp all their calls to WarmUpLoadBalancer.getClient would time out due to the call in ZKDeterministicSubsettingMetadataProvider timing out.
2. Because the service was added to the set of _usedServices after the getClient call, it would never be added
3. Upon shutdown, that service was considered unused and its warm up backup store was deleted (even though it was eventually populated after warmUp).

This is wrong, as we should consider a service used even if the getClient call fails. I fixed this by moving the logic to add the service to the used services before making the getClient call instead of after

Testing done

Updated the unit test testNotDeletingFilesGetClient to test both the case where getClient returns and errors.

Before

Tests fail with

After shutdown there should be just one service, the one that we 'get the client' on
Expected :0
Actual   :1

After

Tests pass

[bohhyang]

Nice catch and test! LGTM.